The present invention relates to the measurement of concentrations of particles in air and, more particularly, to an apparatus and method for detecting particles of submicron size.
A major characteristic in the development of the microelectronic industry is the reduction in size of manufactured components. Each new generation of products requires higher standards for the "clean rooms" in which the products are made in order to keep the percentage of defective products to an economically acceptable level. For a 0.5 micrometer component, the presence of a 0.1 micrometer impurity is "killing", and a 0.01 micrometer particle is enough to alter the behavior of a dielectric film having a 0.02 micrometer diameter.
Atmospheric air contains high concentrations of particles having diameters less than 0.1 micrometers. Although the filters used for air-conditioning clean rooms are highly effective in removing these ultrafine particles, when a mishap occurs in the manufacturing process, it can be expected that a non-negligible fraction of ultrafine particles remains in the atmosphere of the clean room. However, the most likely sources of such particles involve the manufacturing techniques themselves (gas-phase reactions, atomization of fluids, corona discharge, degassing of solids, abrasion, and the like). The current goal in contamination control is to measure very low concentrations, on the order of a few particles per cubic meter, of particles ranging in size from 0.01 micrometers to 10 micrometers. Monitoring must be performed in real time in order to allow rapid intervention in case contamination of the air should occur.
The measurement of contamination in clean rooms has been carried out using optical counters in which a focused light beam constitutes an optical cell which the particles in the air cross one by one. The light scattered individually by each particle is collected by a photosensitive detector. The resultant electrical signals are stored in a multichannel analyzer and the particle size distribution of the sampled aerosol is deduced. Such optical counters using a white light source can be used for particles ranging in size from 0.3 or 0.5 micrometers to approximately 10 micrometers to sample the air at rates of 0.1 or 1 cubic feet per minute, depending on the model of optical counter used. Optical counters employing lasers can be used for detecting particles as small as 0.1 micrometers. Thus, it can be appreciated that further development has been necessary in order to detect particles in the range of 0.01 micrometers to 0.1 micrometers.
It has been known to use the particles as nuclei for the condensation of a supersaturated vapor in an expansion chamber and then to detect with an optical cell the resulting droplets, which are of substantially greater size than the particles alone. However, in one such known device, the expansion chamber in which the droplet formation occurs is combined with the optical cell and, therefore, the measurement relates to the attenuation of the light beam of the optical cell by the cloud of droplets formed. The use of this type of instrument is, thus, limited to concentrations greater than approximately 100 particles per cubic centimeter. In another device employing the particles as condensation nuclei, the droplets formed can be counted individually, with detection being carried out in the same way as in optical counters. However, the sampling rate of the device is very low, approximately 0.3 liters/minute, so that when concentrations of a few particles per cubic meter are involved, sampling times are extremely long.